Rotor for an electric motor and method for producing such a rotor

ABSTRACT

A rotor for an electric motor is provided. The rotor includes a metal cone upon which a plurality of electrical conductors are arranged. A metallic corrosion protection layer is provided that envelopes the metal core and the conductors. The corrosion protection layer is laid around the metal core and the conductors as a solid body and fastened at least to the metal core. The corrosion protection layer is at least a part of a hot-pressing containment for isostatic pressing of the conductors in the metal core. The rotor, which is suitable for a high-speed electric motor for the industrial sector, can be produced at low cost and can be used reliably to compress chemically aggressive industrial process gases.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2010/052730, filed Mar. 4, 2010 and claims the benefitthereof. The International Application claims the benefits of Germanapplication No. 10 2009 012 051.3 DE filed Mar. 6, 2009. All of theapplications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a rotor for an electric motor, comprising ametal core, electrical conductors arranged thereon and a metalliccorrosion protection layer enveloping the metal core and the conductors,the corrosion protection layer being laid as a solid body around themetal core and the conductors and fastened at least to the metal core.The invention furthermore relates to a method for producing a rotor foran electric motor, comprising a metal core and electrical conductorsarranged thereon.

BACKGROUND OF INVENTION

Industrial electric motors for high rotation speeds, for example for aturbocompressor of an industrial installation, contain a solid rotorwhich comprises a steel body and an integrated cage made of copperparts, for example rings and rods. The rods are in this case insertedinto grooves of the metal core, which extend in the axial direction, andare electrically connected to one another by short-circuit ringslikewise made of copper. Such electric motors are also used for thecompression of chemically aggressive process gases in industry, whichmay attack the copper parts so that the working life of these electricmotors is reduced.

In order to protect the sensitive motor parts, it is known from WO2005/124973 A1 to apply a metallic protective layer made of Inconel ontothe rotor. Inconel is a registered trademark of Special MetalsCooperation, New Hartford, N.Y., USA. The protective layer covers thecopper parts and thus protects them from corrosive gases.

SUMMARY OF INVENTION

It is an object of the present invention to provide an economicallyproducible rotor for an electric motor for the industrial sector, whichcan be used reliably for the compression of chemically aggressiveprocess gases in industry.

This object is achieved by a rotor of the type mentioned in theintroduction, in which according to the invention the corrosionprotection layer is laid as a solid body around the metal core and theconductors and is fastened at least to the metal core. Laying thecorrosion protection layer as a solid body around the metal core makesit possible to produce the rotor comparatively simply and economicallycompared with applying corrosion protection metal from the liquid phase.

Advantageously, the corrosion protection layer is fastened not only tothe metal core but also to the conductors. The metal core may be formedin one or a plurality of pieces and consist of one or a plurality ofmetals. The term metal is to be interpreted below as meaning that alloysand metallic mixtures are also to be included. The solid body applied asa corrosion protection layer may be formed in one or a plurality ofpieces, for example fitted as a cylindrical tube over the metal corewith the conductors or applied as a two-piece housing onto the metalcore. The laying may comprise placement or engagement, or more generallyall possible application methods.

The corrosion protection layer may be configured in its geometry as atubular coating over the metal core and the conductors. It delimits thegap between the rotor and the stator of the electric motor. Thecorrosion protection layer may consist of any desired alloy which can beprocessed to form a thin-walled tube and is resistant to the chemicallyaggressive environment to be expected. A high-alloy stainless steel or anickel-based alloy, for example Inconel 625, is particularly suitable. Anickel-based alloy consists primarily of nickel and comprises at leastone further chemical element, which is expediently bonded to the nickelby means of a fusion method.

The fastening of the corrosion protection layer to at least the metalcore may be carried out by welding along a seam or by adhesive bondingor by pressure rolling. Combined methods are also advantageous. Withwelding, additional sealing of the electrical conductors may also becarried out. Purely mechanical methods, such as screwing, may also beenvisaged.

In an advantageous embodiment of the invention, the corrosion protectionlayer is connected to at least the metal core by shrinkage. Amechanically very stable connection to the metal core can be achievedwith such cryoforming. The solid body is in this case advantageouslyexpanded, for example by compressed air, before the shrinkage.

A uniform, two-dimensional and very strong connection of the corrosionprotection layer to the metal core, and in particular also to theconductors, can be achieved if the corrosion protection layer isconnected by hot isostatic pressing to the metal core, and in particularalso the conductors. In hot isostatic pressing (HIP), which may also beregarded as a type of diffusion welding, the electrical conductors andthe part of the metal core which accommodates them are welded andgas-tightly sealed, and optionally also evacuated, in a hot-pressingcontainment, for example a sheet-metal canister, so that the HIP processgas can act on the rotor with the appropriate pressure. Owing to theaction of the pressure force from all sides, the pressing takes placeisostatically.

Subsequently, in a special apparatus, particularly an autoclave, therotor is exposed in a protective gas atmosphere to a very high gaspressure acting on all sides. In addition, the rotor is heated verystrongly, particularly beyond the yield point of the material of theconductors, and thereby compressed. In this way, a material-fitconnection can be achieved between the corrosion protection layer andthe conductors, and in particular also the metal core.

In another advantageous embodiment of the invention, between the metalcore and the corrosion protection layer there is a metallic interlayerwhich is diffused from the material by hot isostatic pressing into themetal core and the corrosion protection layer and holds them together.The underlying concept of this variant of the invention is that theyield point of the superficial material of the metal core is usually atsuch a high temperature that the conductors, which are conventionallymade of copper or a copper alloy, experience damage. By using theinterlayer, for example a nickel layer or an alloy with nickel as itsmain component, welding of the corrosion protection layer and the metalcore can already be achieved by material diffusion at a temperature ofabout 1000° C. The material of the metallic interlayer is in this caseadvantageously such a material as allows diffusion welding of thecorrosion protection layer to the metal core up to a temperature of atmost 1050° C.

According to another advantageous embodiment of the invention, thecorrosion protection layer is at least a part of a hot-pressingcontainment for isostatic pressing of the conductors in the metal core.This approach takes into account the concept that a mechanicallyparticularly strong connection between the conductors and the metal corecan be achieved by hot isostatic pressing. In this case, as describedabove, the mechanical sheet-metal canister is expedient as ahot-pressing containment in order to be able to shield the rotor'selements to be connected here against incoming gas, so that the gaspressure is maintained from the outside.

By the hot isostatic pressing, a connection can be achieved not onlybetween the conductors and the metal core but also between thehot-pressing containment and the metal core, and expediently also theconductors. The hot-pressing containment may also be used as a corrosionprotection layer, if it is made of an appropriate chemically resistantmaterial.

By the enormous pressures during the hot isostatic pressing, theconductors are pressed some way into the grooves of the metal core whichcontain them, so that it is possible for the metal core to protrudebeyond the conductors. This irregular surface shape is transferred tothe hot-pressing containment, which then bears against this surfaceowing to the enoimous pressure. In order to keep the gap between therotor and the stator as small as possible so as to achieve a highefficiency of the electric motor, it is advantageous if the hot-pressingcontainment is machined flat after the hot isostatic pressing so that anaxisymmetric circular surface is obtained.

For gas-tight sealing of the internal space formed by the corrosionprotection layer, it is advantageous if the corrosion protection layeris welded to an element of the metal core on one edge.

Isostatic hot pressing sometimes leads to a temperature profile duringcooling which is unfavorable in terms of the lattice modifications ofthe metallic structure of the metal core. For this reason, at least themajority and in particular more than 90% of the metal core is made of ametal, the structure of which obtains its strength by cooling afterisostatic pressing. Such metals weld poorly, however, in particular to anickel-based alloy. The weldability of the material of the corrosionprotection layer to the material of the metal core can be improved if inthe direction toward the corrosion protection layer, the element towhich the corrosion protection layer is welded comprises a metallicbuffer layer to which the corrosion protection layer is welded and whichdiffers in its composition from the underlying substrate of the metalcore.

The metallic buffer layer facilitates welding of the corrosionprotection layer to the metal core, and may itself be welded to thesubstrate of the metal core.

Simple production of the corrosion protection layer can be achieved ifit is a cylindrical metal tube which is respectively connected,expediently welded, to the metal core at its ends. The corrosionprotection layer is in this case advantageously a cylindrical tubewithout end caps, so that it can be manufactured in one piece and doesnot have to be assembled from a plurality of parts.

Advantageously, the ends are respectively connected, in particularwelded, to a termination ring which holds the conductors axially inposition on the metal core. The termination ring may be machined in onepiece from the metal core or connected to the metal core, for example byhot isostatic pressing.

The termination ring may be an extension of the metal core extendingradially outward, and it expediently slopes radially inward in bothaxial directions. A short-circuit ring may optionally be arrangedbetween the conductors and the termination ring, in which case it isexpediently configured in a plurality of pieces so that it can be laidonto the metal core radially from the outside.

It is furthermore an object of the invention to provide an economicalmethod for producing a corrosion-resistant rotor for an electric motor.

This object is achieved by a method of the type mentioned in theintroduction, in which according to the invention a metallic corrosionprotection layer is laid as a solid body around the metal core and theconductors and is fastened at least to the metal core. Application ofthe corrosion protection layer from liquid can be obviated, and ahigh-strength metal structure of the corrosion protection layer can beachieved in a simple production method.

Advantageously, a hot-pressing containment is laid around the metal coreand the conductors and the conductors are isostatically hot-pressed withthe metal core, at least a part of the hot-pressing containmentsubsequently remaining as a corrosion protection layer on the rotor inorder to protect the rotor during operation of the electric motor. Thehot-pressing containment can therefore fulfill two functions, so thatthe production process can be kept simple. Expediently, the corrosionprotection layer is machined flat following the pressing process, inorder to achieve an axisymmetric surface.

It is furthermore advantageous for an interlayer to be applied betweenthe hot-pressing containment and the metal core, so that the pressingtemperature at which metal diffusion takes place can be kept low. Theinterlayer may consist of nickel, which is applied for example as a foilor electrolytically or chemically onto the hot-pressing containment orthe metal core. The planarization may be carried out by lathing.

According to another advantageous embodiment of the method according tothe invention, the conductors are isostatically hot-pressed with themetal core and the radially outer surface of the rotor is subsequentlymachined flat, then the corrosion protection layer is applied onto theplane surface and fastened to the plane surface. The fastening isexpediently carried out by hot isostatic pressing, in particular byusing the interlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with the aid ofexemplary embodiments, which are represented in the drawings, in which:

FIG. 1 shows a longitudinal section through a rotor of a 10 MW electricmotor for the operation of a turbocompressor,

FIG. 2 shows a cross section through the rotor along the line II-II,

FIG. 3 shows a detail of the rotor of FIG. 1 with the hot-pressingcontainment removed,

FIG. 4 shows the detail of FIG. 3 with a corrosion protection layerapplied,

FIG. 5 shows the corrosion protection layer of FIG. 4 in its form readyfor operation and

FIG. 6 shows a longitudinal section through another rotor of a 15 MWelectric motor.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a detail of a rotor for a 10 MW electric motor for aturbocompressor for the compression of chemically aggressive processgases in an industrial manufacturing process. The compressor is a radialcompressor and is configured for a running speed of at least 14 000 rpm.The rotor comprises a shaft 4, which is represented in a cross sectionin FIG. 2 and which also forms a metal core 6 of the rotor 2.

A multiplicity of grooves 8, which extend parallel to the axialdirections 10 of the rotor 2, are introduced into a central section 18of the metal core 6, which is configured in one piece and is essentiallycylindrical. Electrical conductors 12 in the form of straight copperbars, which are respectively electrically connected to one another attheir axial ends by short-circuit rings 14, are respectively laid in thegrooves 8. Termination rings 16 hold the short-circuit rings 14 and theaxially extending conductors 12 in their axial position on the metalcore 6.

A hot-pressing containment 20 in the form of a metallic cylinder, whichrespectively has an end cap 22 on its two axial ends, is laid around thecentral section 18 of the metal core 6, the electrical conductors 12 andthe short-circuit rings 14 and termination rings 16. The end cap 22 iswelded to the shaft 4, or the metal core 6, as indicated by two filletwelds 24. The end cap 22 is furthermore welded to the cylindrical partof the hot-pressing containment 20, although this is not represented inFIG. 1 for the sake of clarity. By these welds, the hot-pressingcontainment 20 seals the metal core 6 gas-tightly from the surroundingsin the region of the conductors 12.

During the process of producing the rotor 2, it is put in an autoclaveand hot-isostatically pressed at a high pressure and at temperaturesabove 1000° C. The conductors 12 are thereby diffusion-welded to oneanother by means of an interlayer, which is arranged between them andthe metal core 6 in the grooves 8, so that the conductors 12 are placedfirmly in the metal core 6. The conductors 12 are pressed some way intothe grooves 8 by the high pressure, so that between webs 26 of the metalcore 6 the grooves 8 form small depressions into which the hot-pressingcontainment 20 is also pressed in such a way that its cylindricallyshaped part is deformed somewhat from rotational symmetry.

A subsequent method step is explained with the aid of the schematicrepresentation of FIG. 3. The hot-pressing containment 20, including theend caps 22 and the fillet welds 24 is lathed and thereby fully removed,and the metal core 6, the conductors 12, the short-circuit rings 14 andthe termination rings 16 are lathed to radial manufacturing dimensionand thereby machined flat. The copper of the conductors 12 is therebyradially exposed outward.

The next method step is represented with the aid of the diagram in FIG.4. A new hot-pressing containment 28, with its end caps 30 and filletwelds 32, gas-tightly seals the webs 26, conductors 12, short-circuitrings 14 and steel termination rings 16 externally. Arranged between thehot-pressing containment 28 and the conductors 12 and webs 26, there isfurthermore an interlayer 34, for example of nickel, which penetratesboth into the hot-pressing containment 28 and into the conductors 12 andwebs 26 during subsequent hot isostatic pressing and firmly connects theelements together in the capacity of a diffusion weld. In contrast tothe first hot-pressing containment 20, the second hot-pressingcontainment 28 is made from the metal Inconel 695. As explained above,the term “metal” is also intended to include metal alloys.

A metallic buffer layer 36, which is made of a metal that can be weldedwith tolerable outlay to the metal core 6, is furthermore welded ontothe metal core 6. The two buffer layers 36 are arranged on both sides ofthe hot-pressing containment 28, so that the two end caps 30 areconnected via the two buffer layers 36 and the fillet welds 32 to themetal core by welding. The buffer layers 36 may already be provided whenthe fillet welds 24 are applied.

After the subsequent method step of hot isostatic pressing, in which thehot-pressing containment 28 in turn serves as gas-tight shield forbuilding up the application pressure on the metal core 6, as representedin FIG. 5 the hot-pressing containment 28 is lathed to final dimensionand thereby machined flat. It now seals the metal core 6 in the regionof the conductors 12, the short-circuit rings 14 and the terminationrings 16 fully from the external environment, so that the conductors 12are effectively protected against chemical process gases. Thehot-pressing containment 28 now forms a corrosion protection layer 38around the conductors 12. The welding on the metal core 6 and thediffusion welding on the webs 26 and the conductors 12 provides amechanically extremely strong structure which withstands even fastrotations of up to 16 000 revolutions/minute and a rotation speed of upto 300 m/second.

Another exemplary embodiment is represented with the aid of theschematic representation in FIG. 6. The following description isrestricted essentially to the differences from the exemplary embodimentin the preceding figures, to which reference is made in relation to thefeatures and functions that remain the same. Components whichessentially remain the same are in principle numbered with the samereferences, and features not mentioned are included in the followingexemplary embodiment without being described again.

Termination rings 40 are machined integrally from the metal core 6 asradial extensions, which slope radially inward in both axial directions10. The axial boundaries are thereby formed for short-circuit rings 42which are configured in two pieces, i.e. as half-rings, and are laidaround the metal core 6. The conductors 12 are inserted between the webs26, in particular with an interlayer, and a corrosion protection layer44 which is made of a nickel-based alloy and has a cylindrical shapewithout end caps is laid around the conductors 12 and the webs 26. Atits axial ends, the corrosion protection layer 44 is welded to thetermination rings 40, i.e. the metal core 6 itself, buffer layers 36again being advantageous in this case. An interlayer 34 facilitates theisostatic hot pressing since by virtue of it, as in the precedingexemplary embodiment, diffusion welding can already take place betweenthe corrosion protection layer 44 and the metal core 6 at a temperaturewhich lies below the yield point of the metal of the metal core 6.

In a single hot isostatic pressing method step, the conductors 12 arenow firmly connected mechanically to the metal core 6 and the corrosionprotection layer 44 is firmly connected to the metal core 6 and theconductors 12. As an alternative, it is possible to carry out theconnection in two separate method steps, in which case the conductors 12are initially connected to the metal core 6 with the aid of ahot-pressing containment 20, the hot-pressing containment 20 issubsequently removed as in the preceding exemplary embodiment, the webs26 and conductors 12 are machined flat, and as a subsequent step thecorrosion protection layer 44 is applied onto the conductors 12 and webs26 and connected thereto.

The invention claimed is:
 1. A method for producing a rotor for anelectric motor, the rotor comprising a metal core and a plurality ofelectrical conductors arranged on the metal core, the method comprising:laying a metallic corrosion protection layer as a solid body around themetal core and the conductors, and fastening the corrosion protectionlayer at least to the metal core, wherein said laying the metalliccorrosion protection layer as a solid body around the metal core and theconductors comprises: laying a metallic hot-pressing containment aroundthe metal core and the conductors, subsequently isostaticallyhot-pressing the conductors with the metal core, wherein subsequent tosaid isostatic hot-pressing, at least a part of the hot-pressingcontainment remains as the corrosion protection layer on the rotor toprotect the rotor during operation of the electric motor.
 2. The methodas claimed in claim 1, wherein the conductors are isostaticallyhot-pressed with the metal core and a radially outer surface of therotor is subsequently machined flat, after which the corrosionprotection layer is applied onto the plane surface and fastened to theplane surface.
 3. The method as claimed in claim 1, wherein thefastening is carried out by hot isostatic pressing.
 4. A rotor for anelectric machine produced by the method of claim 1.